1. Field of the Invention
The present invention relates to closures for liquid product containers and, methods and apparatus for making the same. More specifically, the present invention relates to molded synthetic plastic closures which resemble the appearance of natural cork and molding techniques for making the same.
2. Description of Prior Art
It is known heretofore to manufacture articles from foamed thermoplastic materials by injection or extrusion of polyurethane, polystyrene or polyolefins. In these cases, the thermoplastic material may be expanded from a granular form to assume a porous, solidified foam-like structure by the action of various propellants or agents for expanding or blowing the materials. The blowing agents, in accordance with normal practice, are usually gases or gas generating substances that have been dissolved or otherwise intimately incorporated within the thermoplastic resinous material while it is in an unexpanded granular form. The application of heat to such a mixture causes the blowing agent to be released or thermally expanded or both, while the thermoplastic material is attaining a foaming temperature at which it is sufficiently softened and yieldable to permit the pressure of the expanding blowing agent to expand it into the desired foam structure.
In a special case of extrusion forming of foamed thermoplastic materials, it is also known to decorate the surface of articles so produced to resemble natural wood by imparting to the surface a parallel grain structure. Such decoration is accomplished by extruding the material through a die having, in particular, a multiple number of apertures to form a corresponding number of strands of expanding resin, collecting these strands in parallel relationship, and then coalescing them into a body of any desired length. Contacting surfaces of the strands create parallel grain lines which remain clearly visible in the surface of the finished articles and resemble natural wood grain.
In a special case of injection molding of foam thermoplastics, it is known to mold articles by injecting into an enclosed cavity a measured amount of expandable thermoplastic resin so that the cavity is not completely filled by the unexpanded material but becomes filled as the material expands. As expansion occurs, the softened resin is forced by a gaseous blowing agent into contact with the relatively cool walls of the mold cavity where the portion of the resin in direct contact with the walls is solidified and forms a dense outer layer or skin conforming to the shape of the cavity and functioning as a barrier, thereby preventing the escape of the gaseous blowing agent from the article and causing gas bubbles to be entrapped in the article's interior region. Thus, articles molded in this manner are characterized by a porous, foam-like interior and a dense outer shell or skin having the same general size and shape as the mold cavity and which are essentially impermeable to most substances.
By employing certain highly flexible thermoplastic materials, for example polyolefin copolymers, such as ethylene vinyl acetate, in the previously described injection molding process, the resulting foamed articles will exhibit elastic properties closely resembling similar products manufactured from other natural and synthetic elastomers. This flexibility and resiliency plus the presence of an essentially impermeable skin allows these same articles if molded into the proper size and shape, to function as closures for liquid product containers by being compressed and fitted into the container opening, then being allowed to expand, filling the opening and creating a continuous seal about its perimeter.
Therefore, such articles would theoretically be useful as replacements for similar closure devices manufactured of more familiar materials, such as natural cork used in containers of medicine, wine and spirits. In reality, efforts at replacing natural cork closures in liquid product containers, especially containers for wine products, by manufacturing and utilizing closures from injection molded foamed thermoplastics have not been very successful. This has been due primarily, it is believed, to the lack of aesthetically created consumer appeal for such closures, and the relationship of the dense outer skin to the porous inner core which dense outer skin, while necessary to make the closure essentially impermeable, can be a detriment to the sealing function of these articles, particularly when leakage channels are formed on the dense outer skin during the sealing of these articles.
Since such prior art closures must be compressed to be fitted into liquid product containers there is a tendency for the dense outer layer of the closures to wrinkle and form longitudinal channels in the outer surface of the foamed articles. Such channels could form leakage paths for the container contents or allow the ingress of substances harmful to the product being contained, such as ambient air in the case of the medicine or food products. Tests have shown that these channels are caused primarily by the presence of relatively large void spaces between the dense outer skins and the porous inner core of the foamed articles, their presence being the result of a previously unrecognized phenomena occuring during the molding of the articles. Since the dense outer skin has a much higher resistance to compression than the porous inner core, it tends to deflect into any available void space, creating a channel on the outer surface of the closure that could result in a disruption of the otherwise continuous perimeter seal.
In addition to the foregoing problems, prior art molded closures have not been uniform and consistent in design and performance. Thus, such prior art closures have not been found readily acceptable as a replacement for similar closure devices, such as natural cork used in containers of medicine, wine and spirits. It is well known that natural cork contains various holes and crooks and many times will crumble and break when inserted or withdrawn from the product container.